This invention relates to motive power battery casings and the use of blow molding procedures in their fabrication.
Motive power battery cases of the type described hereinafter tend to be fairly long (about 30 inches) and rectangular in cross-section (about 5.times.6 inches). For many applications, a plurality of motive power batteries are grouped together in order to provide sufficient electrical power and capacity. It is, therefore, important that reasonable dimensional tolerances are maintained as to outside dimensions and inside dimensions so that the grouping of batteries will be reasonably compact, so that the plates may be properly fit within the casings and so that the casings can be properly heat sealed at their tops. Furthermore, battery cases must be sufficiently tough and impact resistant since they are often used in moving vehicles where they might well be subjected to severe jostling and impact with one another and with the housing containing them.
Conventional practice has been to prepare motive power battery cases through the use of injection or rotational molding. These procedures permit tolerances to be maintained with a relatively high degree of accuracy. Blow molding, however, has not been used primarily because it has not been possible to maintain reasonably accurate tolerances with this process. As a result, blow molding techniques have been used primarily in applications where tolerances are not particularly important and those skilled in the art would not expect the blow molding process to be satisfactory for the purpose of preparing motive power battery cases. This has been unfortunate because there are several advantages associated with blow molding as compared with injection or rotational molding. For one thing, it is possible to use materials in the blow molding process which are much tougher. This would permit the thickness and hence the weight of the battery casings to be lighter and more suitable for transportation applications. Secondly, blow molding is a much less costly process than injection or rotational molding. Furthermore, blow molding is a low stress process while the other processes are relatively high stress in nature. Because of this and because of the differences in the materials used in the two processes, the products made in each exhibit dramatic difference in their resistance to stress cracking, the blow molded product being far superior in this regard. Finally, the blow molding process offers a much higher production rate.
The primary obstacle to the use of blow molding in the production of motive power battery cases relates to the rectangular shape which is optimal for this application. The parison formed in the blow molding process has a generally cylindrical shape and when it is blown into a rectangular shape, the corners of the product will be relatively thin and weak. As a result, it is necessary to either make the entire product overly thick or to find a way in which to avoid undue thinness in the corners and joints where the wall sections meet.
It is an object of this invention to produce a battery casing of great toughness and within close tolerances through the use of a blow molding process.
It is a further object of this invention to produce a battery casing having relatively thinner walls than has heretofore been possible in order to produce a relatively lightweight casing at low cost.
Briefly described, the invention is carried out by making extrusion notches in a die in order to produce a parison having longitudinal ribs of greater thickness than the walls of the parison in general. The parison is aligned so that these ribs will ultimately become the corners and joints where the walls and bottom meet so as to maintain sufficient thickness in these areas. The mold used is provided with a unique venting arrangement to assure the full and complete formation of the rectangularly shaped casing.
Also briefly described, the battery casing produced as a part of this invention is a blow molded rectangularly shaped container having minimum wall joint thickness of 0.025 inches while maintaining maximum overall wall thickness of 0.140 inches. A collar section for heat sealing is provided having wall joint thickness of at least 0.070 inches and overall wall thickness not exceeding 0.140 inches. Similarly, the bottom joints and the corners of the container have a minimum thickness of 0.025 inches.